Apparatus and methods for input protection for power converters

ABSTRACT

Systems and methods in accordance with this invention provide a power converter including an input signal terminal, a first output signal at a first output signal terminal, and a controller. The controller is adapted to switch the first output signal from a first value to a second value, measure a voltage at the input signal terminal as a function of time, set a flag to a first flag value if the measured voltage falls below a predetermined value within a first predetermined time interval after the first output signal has been switched from the first value to the second value, otherwise set the flag to a second flag value, and save the flag in a memory. Numerous other aspects are also provided.

BACKGROUND

This invention relates to power converters. More particularly, thisinvention relates to apparatus and methods for input protection forpower converters, such as DC-DC converters, AC-DC converters, DC-ACinverters, variable frequency drives, and other similar powerconverters.

SUMMARY

In a first aspect of the invention, a power converter is providedincluding an input signal terminal, a first output signal at a firstoutput signal terminal, and a controller. The controller is adapted to:(a) switch the first output signal from a first value to a second value,(b) measure a voltage at the input signal terminal as a function oftime, (c) set a flag to a first flag value if the measured voltage fallsbelow a predetermined value within a first predetermined time intervalafter the first output signal has been switched from the first value tothe second value, (d) set the flag to a second flag value if themeasured voltage does not fall below a predetermined value within thefirst predetermined time interval after the first output signal has beenswitched from the first value to the second value, and (e) save the flagin a memory.

In a second aspect of the invention, a method of operating a powerconverter is provided, the power converter including an input signalterminal, a first output signal terminal and a second output signalterminal. The method includes: (a) providing a first output signal atthe first output signal terminal, (b) switching the first output signalfrom a first value to a second value, measuring a voltage at the inputsignal terminal as a function of time, (c) setting a flag to a firstflag value if the measured voltage falls below a predetermined valuewithin a first predetermined time interval after the first output signalhas been switched from the first value to the second value, (d) settingthe flag to a second flag value if the measured voltage does not fallbelow the predetermined value within the first predetermined timeinterval after the first output signal has been switched from the firstvalue to the second value, and (e) saving the flag in a memory.

Other features and aspects of the present invention will become morefully apparent from the following detailed description, the appendedclaims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention can be more clearly understood fromthe following detailed description considered in conjunction with thefollowing drawings, in which the same reference numerals denote the sameelements throughout, and in which:

FIG. 1 is a block diagram of a system including an example embodiment ofa power converter in accordance with this invention;

FIG. 2 is a flow diagram of an example test process in accordance withthis invention; and

FIG. 3 is a flow diagram of an example drive fault process in accordancewith this invention.

DETAILED DESCRIPTION

A power converter, such as a DC-DC converter, AC-DC converter, DC-ACinverter, variable frequency drive (“VFD”), or other similar powerconverter, is typically installed with an input protection device (e.g.,a circuit breaker) designed to interrupt or disconnect power from thepower converter under certain conditions (e.g., a short circuit, anovercurrent, etc.). Some input protection devices include a controlinput that may be coupled to an electrical control signal to open theinput protection device and disconnect power from the protected circuiteven if a short circuit or overcurrent condition has not occurred.

Thus, some power converters provide a control signal output that isspecified to be connected to the control input of an input protectiondevice, and that may be triggered in the event of a predeterminedcondition in the power converter. For example, in a VFD, thepredetermined condition may be a catastrophic internal failure, such asexcessive power losses in the VFD, excessive level of reactive inputcurrent to the VFD, arcing within the VFD, etc.

Unfortunately, some installers may not properly install a protectiondevice with the VFD. For example, an inexperienced installer may notinstall any input protection device, may install an incorrect type ofinput protection device that does not include a control input, or maynot connect the control signal output of the VFD to the control input ofthe input protection device.

Further, even if the input protection device is initially installedcorrectly, as a result of subsequent tampering, the control signaloutput of the VFD may become disconnected from the control input of theinput protection device. In such circumstances, the VFD is unable tocontrol the input protection device, and thus is unable to disconnectthe VFD from the power source.

To overcome these problems, systems and methods in accordance with thisinvention provide a VFD that includes an input protection test functionand a drive fault function. The input protection test function, whichmay be initiated at any time by a user, performs a test to open theinput protection device to disconnect power from the VFD. A Drive RunInhibit (“DRI”) flag is set based on the test results (e.g., pass=FALSE,and fail=TRUE), and is saved in non-volatile memory. The VFD ispermitted to run only if the saved DRI flag value is FALSE, and is notpermitted to run if the saved DRI flag value is TRUE. In this regard,the VFD may operate only if the VFD is able to open the input protectiondevice to disconnect power to the VFD.

The drive fault function may be initiated on the occurrence of apredetermined condition in the VFD (e.g., a catastrophic internalfailure in the VFD, such as excessive power losses, excessive level ofreactive input current, internal arcing, or some other condition). Thedrive fault function also performs the test described above to open theinput protection device to disconnect power from the VFD, and alsostores the DRI flag in memory. In addition, if the test fails, the VFDmay open an upstream input protection device to disconnect power to theVFD.

For simplicity, the remaining text will describe the invented methodsand apparatus using a VFD power converter. Persons of ordinary skill inthe art will understand, however, that the invented methods andapparatus may be used with other power converters, such as DC-DCconverters, AC-DC converters, DC-AC inverters, or other similar powerconverter.

Referring now to FIG. 1, an example system that includes a VFD inaccordance with this invention is described. In particular, system 10includes a VFD 12, an AC power source 14, a first circuit breaker 16, asecond circuit breaker 18 and an AC motor 20. Persons of ordinary skillin the art will understand that system 10 may include components otherthan or in addition to the components illustrated in FIG. 1.

AC power source 14 may be an electric utility, a generator, or othersimilar source of AC power. In the illustrated embodiment, AC powersource 14 provides three phase (φ1-φ3) power. Persons of ordinary skillin the art will understand that AC power source 14 alternatively mayprovide one, two, or more than three phase power.

First circuit breaker 16 and second circuit breaker 18 each may includea trip mechanism that opens electrical contacts (not shown) in responseto an overcurrent or short-circuit fault to stop the flow of current toVFD 12 and AC motor 20. In addition, first circuit breaker 16 includes acontrol input terminal 22 adapted to cause the trip mechanism to openthe electrical contacts in first circuit breaker 16 when an electricalsignal at control input terminal 22 switches from a first value (e.g.,0V or “OFF”) to a second value (e.g., 5V or “ON”). Likewise, secondcircuit breaker 18 includes a control input terminal 24 adapted to causethe trip mechanism to open the electrical contacts in second circuitbreaker 18 when an electrical signal at control input terminal 24switches from the first value to the second value.

For example, first circuit breaker 16 and second circuit breaker 18 eachmay be a Type GMSG vacuum circuit breaker by Siemens Industry, Inc.,Wendell, N.C. Persons of ordinary skill in the art will understand thatother circuit breakers may be used, and that first circuit breaker 16and second circuit breaker 18 may both be the same type of circuitbreaker, or may be different types of circuit breakers. In addition,persons of ordinary skill in the art will understand that first circuitbreaker 16 and/or second circuit breaker 18 alternatively may be acontactor used to switch power ON and OFF to VFD 12.

First circuit breaker 16 is typically located near VFD 12, and maycommonly be referred to as a “drive input breaker.” Accordingly, firstcircuit breaker 16 will also be referred to herein as drive inputbreaker 16. Persons of ordinary skill in the art will understand thatfirst circuit breaker 16 may be separate from VFD 12 (as shown in FIG.1), or may be included as part of VFD 12.

Second circuit breaker 18 is typically located “upstream” from driveinput breaker 16 and VFD 12, and may commonly be referred to as an“upstream breaker.” Accordingly, second circuit breaker 18 will also bereferred to herein as upstream breaker 18. Second circuit 18 typicallymay provide protection to multiple circuits in addition to the circuitincluding drive input breaker 16 and VFD 12.

VFD 12 includes input terminals 26(a)-26(c) coupled to output terminalsof drive first circuit breaker 16, a first output terminal 28 providingan output signal TRIP coupled to control input terminal 22 of firstcircuit breaker 16, and a second output terminal 30 providing an outputsignal FAULT coupled to control input terminal 24 of second circuitbreaker 18. In addition, VFD 12 includes a controller 32, a driveelectronics circuit 34 and a memory 36. Controller 32 is coupled toinput terminals 26(a)-26(c), first output terminal 28, second outputterminal 30, drive electronics circuit 34 and memory 36. Persons ofordinary skill in the art will understand that VFD 12 may includecomponents other than or in addition to the components illustrated inFIG. 1.

Controller 32 may be a microprocessor controller, a programmable logiccontroller, a mainframe computer, a personal computer, a handheldcomputer, or other similar processor that may be used to control theoperation of drive electronics circuit 34. Drive electronics circuit 34may include conventional VFD electronics circuits that convert an ACinput signal at input terminals 26(a)-26(c) to an AC output signalcoupled to AC motor 20. Memory 36 may be a non-volatile memory, such asa magnetic disc memory, an optical disc memory, a hard drive, a floppydisc, a flash memory, or other similar memory.

In accordance with this invention, controller 32 includes an inputprotection test process and a drive fault process. The input protectiontest process, which may be initiated at any time by a user of VFD 12,performs a test to open first circuit breaker 16 to disconnect powerfrom VFD 12. If the test is successful, controller 32 sets a DRI flagvalue to FALSE. If, however, the test is unsuccessful, controller 32sets the DRI flag value to TRUE. The DRI flag is stored in memory 36.VFD 12 is permitted to run if DRI=FALSE, indicating that VFD 12 is ableto open first circuit breaker 16. IF DRI=TRUE, controller 32 will notpermit VFD 12 to run.

The drive fault process may be initiated by controller 32 on theoccurrence of a predetermined condition (e.g., a catastrophic internalfailure, such as excessive power losses in VFD 12, excessive level ofreactive input current to VFD 12, arcing within the VFD, or some othercondition) to open drive input breaker 16 to disconnect power from VFD12. The drive fault process also performs the test process describedabove to open first circuit breaker 16 to disconnect power from VFD 12,and also stores the DRI flag in memory 36. In addition, as described inmore detail below, if the test is unsuccessful, controller 32 may opensecond circuit breaker 18 to disconnect power from first circuit breaker16 and VFD 12.

Referring now to FIGS. 1 and 2, an example test process 40 in accordancewith this invention is described. Test process 40 may be initiated by auser by issuing a test command to controller 32, such as via a hardwareand/or a software switch, or by other similar method. At step 42, inresponse to the test command, controller 32 switches TRIP signal atfirst output terminal 28 from a first value, such as OFF (e.g., 0V) to asecond value, such as ON (e.g., 5V). The TRIP signal is coupled tocontrol terminal 22 of first circuit breaker 16, and thus when TRIPsignal switches from OFF to ON, first circuit breaker 16 should open todisconnect power from VFD 16.

To determine if first circuit breaker 16 has opened, at step 44,controller 32 monitors input voltages V_(IN)(φ1), V_(IN)(φ2) andV_(IN)(φ3) at input terminals 26(a), 26(b) and 26(c), respectively, as afunction of time. If TRIP signal is properly connected to controlterminal 22 of first circuit breaker 16, and if first circuit breaker 16is properly functioning, input voltages V_(IN)(φ1), V_(IN)(φ2) andV_(IN)(φ3) each should decrease to 0V within a relatively short timeinterval after TRIP signal has been switched from OFF to ON.

Thus, at step 46, controller 32 determines if input voltages V_(IN)(φ1),V_(IN)(φ2) and V_(IN)(φ3) each are less than or equal to a predeterminedvalue V_(MIN) within a first predetermined time interval T_(MAX) afterTRIP signal has been switched from OFF to ON. Predetermined valueV_(MIN) may be between about 15V and about 0V, or some other value thatmay be used to determine that first circuit breaker 16 has disconnectedpower from VFD 12. First predetermined time interval T_(MAX) may bebetween about 1 second and about 1 microsecond, or some other timeinterval for first circuit breaker 16 to disconnect power from VFD 12after TRIP signal has been switched from OFF to ON.

If input voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) are each lessthan or equal to predetermined value V_(MIN) within first predeterminedtime interval T_(MAX) after TRIP signal has been switched from OFF toON, the process proceeds to step 48, and controller 32 sets the DRI flagvalue to FALSE. At step 50, controller 32 stores the DRI flag in memory36. At step 52, controller 32 optionally may display a message to theuser indicating that the test has passed. Process 40 then ends withinput voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) each less than orequal to predetermined value V_(MIN), and with DRI=FALSE saved in memory36. As a result, controller 32 does not prevent VFD 12 from runningbased on the input protection test results.

Referring again to step 46, if controller 32 determines that one or moreof input voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) are not lessthan or equal to predetermined value V_(MIN) within first predeterminedtime interval T_(MAX) after TRIP signal has been switched from OFF toON, the process proceeds to step 54, and controller 32 sets the DRI flagvalue to TRUE. At step 50, controller 32 stores the DRI flag in memory36. At step 56, controller 32 optionally may display a message to theuser indicating that the test has failed. Process 40 then ends withDRI=TRUE saved in memory 36. As a result, controller 32 prevents VFD 12from running based on the input protection test results.

In example process 40 described above, controller 32 monitors each ofinput voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) to determine if allthree input voltage are less than or equal to predetermined valueV_(MIN) within first predetermined time interval T_(MAX) after TRIPsignal has been switched from OFF to ON. Persons of ordinary skill inthe art will understand that controller 32 alternatively may monitorfewer than all input voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3).

For example, at step 44, controller 32 may monitor input voltageV_(IN)(φ2) and at step 46, controller 32 alternatively may determine ifinput voltage V_(IN)(φ2) is less than or equal to predetermined valueV_(MIN) within first predetermined time interval T_(MAX) after TRIPsignal has been switched from OFF to ON, and may set the DRI flag valueaccordingly. Other alternatives also may be used.

Referring now to FIGS. 1 and 3, an example drive fault process 60 inaccordance with this invention is described. Drive fault process 60 maybe initiated by controller 32 on the occurrence of a predeterminedcondition (e.g., a catastrophic internal failure, such as excessivepower losses in VFD 12, excessive level of reactive input current to VFD12, arcing within VFD 12, or some other condition).

At step 62, in response to the drive fault command, controller 32switches TRIP signal at first output terminal 28 from a first value,such as OFF (e.g., 0V) to a second value, such as ON (e.g., 5V). Whenthe TRIP signal switches from OFF to ON, first circuit breaker 16 shouldopen to disconnect power from VFD 16.

To determine if first circuit breaker 16 has opened, at step 64,controller 32 monitors input voltages V_(IN)(φ1), V_(IN)(φ2) andV_(IN)(φ3) at input terminals 26(a), 26(b) and 26(c), respectively, as afunction of time. If TRIP signal is properly connected to controlterminal 22 of first circuit breaker 16, and if first circuit breaker 16is properly functioning, input voltages V_(IN)(φ1), V_(IN)(φ2) andV_(IN)(φ3) each should decrease to 0V within a relatively short timeperiod after TRIP signal has been switched from OFF to ON.

Thus, at step 66, controller 32 determines if input voltages V_(IN)(φ1),V_(IN)(φ2) and V_(IN)(φ3) each are less than or equal to predeterminedvalue V_(MIN) within first predetermined time interval T_(MAX) afterTRIP signal has been switched from OFF to ON. If input voltagesV_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) are each less than or equal topredetermined value V_(MIN) within first predetermined time intervalT_(MAX) after TRIP signal has been switched from OFF to ON, the processproceeds to step 68, and controller 32 sets the DRI flag value to FALSE.At step 70, controller 32 stores the DRI flag in memory 36. Process 60then ends with input voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) eachless than or equal to predetermined value V_(MIN), and with DRI=FALSEsaved in memory 36.

Referring again to step 66, if controller 32 determines that one or moreof input voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) are not lessthan or equal to predetermined value V_(MIN) within first predeterminedtime interval T_(MAX) after TRIP signal has been switched from OFF toON, the process proceeds to step 72, and controller 32 sets the DRI flagvalue to TRUE. At step 74, controller 32 stores the DRI flag in memory36.

In some instances, drive input breaker 16 may not be able to disconnectpower to VFD 12 within T_(MAX), but may be able to do so within a longertime interval. Accordingly, at step 76, controller 32 continues tomonitor input voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) at inputterminals 26(a), 26(b) and 26(c), respectively, as a function of time,and determines if input voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3)each are less than or equal to predetermined value V_(MIN) within asecond predetermined time T_(FLT) after TRIP signal has been switchedfrom OFF to ON. Second predetermined time interval T_(FLT) may bebetween about 1 second and about 5 seconds, or some other time intervallonger than T_(MAX). If so, process 60 ends with input voltagesV_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) each less than or equal topredetermined value V_(MIN), and with DRI=TRUE saved in memory 36.

If, however, at step 76, controller 32 determines that one or more ofinput voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3) are not less thanor equal to predetermined value V_(MIN) within second predetermined timeinterval T_(FLT) after TRIP signal has been switched from OFF to ON, theprocess proceeds to step 78, and controller 32 switches the FAULT signalat second output terminal 30 from a first value, such as OFF (e.g., 0V)to a second value, such as ON (e.g., 5V), which causes second circuitbreaker 18 to open to disconnect power to first circuit breaker 16 andVFD 12.

In this regard, if first circuit breaker 16 is unable to disconnectpower to VFD 12 under the longer time interval T_(FLT), this mayindicate a serious safety problem. As a result, controller 32 openssecond circuit breaker 18 to prevent serious damage or injury. Inaddition, at step 80, controller 32 issues a Drive Fault Input BreakerOpen Failure warning to indicate the nature of the fault. Process 60then ends with DRI=TRUE saved in memory 36.

In example process 60 described above, controller 32 monitors each ofinput voltages V_(IN)(φ1), V_(IN)(φ2) and V_(IN)(φ3), and determines ifeach input voltage is less than or equal to predetermined value V_(MIN)within first predetermined time interval T_(MAX) after TRIP signal hasbeen switched from OFF to ON. Persons of ordinary skill in the art willunderstand that drive fault functions in accordance with this inventionalternatively may monitor fewer than all input voltages V_(IN)(φ1),V_(IN)(φ2) and V_(IN)(φ3). For example, at step 64, controller 32alternatively may monitor input voltages V_(IN)(φ1) and V_(IN)(φ3), andat step 66, controller 32 alternatively may determine if each of inputvoltages V_(IN)(φ1) and V_(IN)(φ3) is less than or equal topredetermined value V_(MIN) within first predetermined time intervalT_(MAX) after TRIP signal has been switched from OFF to ON, and may setthe DRI flag value accordingly. The same alternative example applies tostep 76. Other alternatives also may be used.

The foregoing merely illustrates the principles of this invention, andvarious modifications can be made by persons of ordinary skill in theart without departing from the scope and spirit of this invention.

The invention claimed is:
 1. A power converter comprising: an inputsignal terminal; a first output signal at a first output signalterminal; a first circuit breaker having an output terminal coupled tothe input signal terminal and a control input terminal coupled to thefirst output signal terminal; and a controller including an inputprotection test process performing an input protection test, wherein,for the input protection test process, the controller is adapted to:switch the first output signal from a first value to a second value;measure a voltage at the input signal terminal as a function of time;set a flag to a first flag value if the measured voltage falls below apredetermined value within a first predetermined time interval after thefirst output signal has been switched from the first value to the secondvalue; set the flag to a second flag value if the measured voltage doesnot fall below the predetermined value within the first predeterminedtime interval after the first output signal has been switched from thefirst value to the second value; and save the flag in a memory, displaya message indicating that the input protection test has passed when theflag is set to the first flag value, and permit the power converter torun based on a passed input protection test, and display a messageindicating that the input protection test has failed when the flag isset to the second flag value, and prevent the power converter fromrunning based on a failed input protection test, wherein the controllerfurther includes a drive fault function, the drive fault functionperforming the input protection test process and, when the inputprotection process test has failed, the controller is adapted to: causea second circuit breaker to open to disconnect power to the firstcircuit breaker and the power converter.
 2. The power converter of claim1, wherein: the first circuit breaker is adapted to open when the firstoutput signal switches from the first value to the second value.
 3. Thepower converter of claim 1, wherein: the first circuit breaker includesa breaker input terminal; the breaker input terminal is coupled to amulti-phase input signal; and the controller is adapted to measure avoltage at each phase of the input signal as a function of time.
 4. Thepower converter of claim 3, wherein the controller is adapted to: setthe flag to the first value if the measured voltage of each phase of themulti-phase input signal falls below the predetermined value within thefirst predetermined time interval after the first output signal has beenswitched from the first value to the second value; and set the flag tothe second value if the measured voltage of any phase of the multi-phaseinput signal does not fall below the predetermined value within thefirst predetermined time interval after the first output signal has beenswitched from the first value to the second value.
 5. The powerconverter of claim 1, wherein the predetermined value is between about15V and about OV.
 6. The power converter of claim 1, wherein the firstpredetermined time interval is between about 1 second and about 1microsecond.
 7. The power converter of claim 1, wherein the controllerswitches the first output signal from the first value to the secondvalue in response to either a test command or a drive fault.
 8. Thepower converter of claim 1, further comprising a second output signal ata second output signal terminal, wherein the controller is furtheradapted to switch the second output signal from a first value to asecond value if the measured voltage does not fall below thepredetermined value within a second predetermined time interval afterthe first output signal has been switched from the first value to thesecond value.
 9. The power converter of claim 8, wherein: the firstcircuit breaker includes a breaker input terminal; the breaker inputterminal is coupled to an output terminal of the second circuit breaker;the second output signal terminal of the power converter is coupled to acontrol input terminal of the second circuit breaker; and the secondcircuit breaker is adapted to open when the second output signalswitches from the first value to the second value.
 10. The powerconverter of claim 8, wherein the second predetermined time interval isbetween about 1 second and about 5 seconds.
 11. The power converter ofclaim 1, wherein the power converter comprises one or more of a DC-DCconverter, an AC-DC converter, an DC-AC inverter, and a variablefrequency drive (“VFD”).
 12. A method of operating a power convertercomprising a first circuit breaker, an input signal terminal, a firstoutput signal terminal and a second output signal terminal, the methodcomprising: providing an output terminal of the first circuit breakercoupled to the input signal terminal and a control input terminal of thefirst circuit breaker coupled to the first output signal terminal;providing a first output signal at the first output signal terminal;switching the first output signal from a first value to a second value;measuring a voltage at the input signal terminal as a function of time;setting a flag to a first flag value if the measured voltage falls belowa predetermined value within a first predetermined time interval afterthe first output signal has been switched from the first value to thesecond value; setting the flag to a second flag value if the measuredvoltage does not fall below the predetermined value within the firstpredetermined time interval after the first output signal has beenswitched from the first value to the second value; and saving the flagin a memory, displaying a message indicating that an input protectiontest has passed when the flag is set to the first flag value, andpermitting the power converter to run based on a passed input protectiontest, displaying a message indicating that the input protection test hasfailed when the flag is set to the second flag value, preventing thepower converter from running based on a failed input protection test,and opening a second circuit breaker to disconnect power to the firstcircuit breaker and the power converter when the input protection testhas failed.
 13. The method of claim 12, wherein the first circuitbreaker is adapted to open when the first output signal switches fromthe first value to the second value.
 14. The method of claim 12, furthercomprising coupling a breaker input terminal of the first circuitbreaker to a multi-phase input signal, wherein measuring the voltagecomprises measuring a voltage at each phase of the input signal as afunction of time.
 15. The method of claim 14, further comprising:setting the flag to the first value if the measured voltage of eachphase of the multi-phase input signal falls below the predeterminedvalue within the first predetermined time interval after the firstoutput signal has been switched from the first value to the secondvalue; and setting the flag to the second value if the measured voltageof any phase of the multi-phase input signal does not fall below thepredetermined value within the first predetermined time interval afterthe first output signal has been switched from the first value to thesecond value.
 16. The method of claim 12, wherein the predeterminedvalue is between about 15V and about OV.
 17. The method of claim 12,wherein the first predetermined time interval is between about 1 secondand about 1 microsecond.
 18. The method of claim 12, further comprisingswitching the first output signal from the first value to the secondvalue in response to either a test command or a drive fault.
 19. Themethod of claim 12, further comprising: providing a second output signalat the second output signal terminal; and switching the second outputsignal from a first value to a second value if the measured voltage doesnot fall below the predetermined value within a second predeterminedtime interval after the first output signal has been switched from thefirst value to the second value.
 20. The method of claim 19, furthercomprising: coupling a breaker input terminal of the first circuitbreaker to an output terminal of the second circuit breaker; andcoupling the second output signal terminal of the power converter to acontrol input terminal of the second circuit breaker, wherein the secondcircuit breaker is adapted to open when the second output signalswitches from the first value to the second value.
 21. The method ofclaim 19, wherein the second predetermined time interval is betweenabout 1 second and about 5 seconds.
 22. The power converter of claim 12,wherein the power converter comprises one or more of a DC-DC converter,an AC-DC converter, an DC-AC inverter, and a variable frequency drive(“VFD”).